Al Qaeda turned more to pony-express-style communications after bin Laden learned that his radio communications in Tora Bora, Afghanistan, where he evaded capture, had been intercepted by the U.S. National Security Agency. The United States never heard his voice again, unless it was on one of his released audio messages aimed at followers and enemies alike.

“They retreated to couriers after Tora Bora, which is why it took as long as it did to track UBL down,” said former CIA officer Bart Bechtel, referring to the CIA’s code name for bin Laden.

A military intelligence source said the Taliban is so aware that the United States can eavesdrop that commanders sometimes mock Americans during radio and phone calls.

Finding couriers became as important to U.S. intelligence as intercepting a critical email or cellphone call.

A former intelligence officer who served in Iraq and Afghanistan said finding couriers became standard operating procedure because they could lead the U.S. to the top-ranking insurgents.

It was at an Internet cafe near Baghdad that intelligence identified a courier tied to Abu Musab Zarqawi, a ruthless al Qaeda operative in Iraq. This go-between led trackers to Zarqawi’s spiritual adviser, who was followed to Zarqawi’s hide-out near Baqouba. A U.S. F-16 airstrike killed the terrorist master in 2006.

“Couriers are a huge pain and usually are smart enough to minimize their footprint and time on the phone or in a cafe,” the former intelligence officer said. “It is really hard to pick them up.”

In tracking down and killing bin Laden, the CIA first learned of the existence of a special messenger from interrogations of al Qaeda captives. It was not until 2007 that the agency learned his name and then, last year, tracked him to the walled compound in Abbottabad. There, in the early hours of May 2, U.S. Navy SEALs killed bin Laden.

A senior intelligence official told reporters at the Pentagon that bin Laden relied heavily on a courier network.

“That’s precisely what led us to the compound,” he said. “So it is probable that the couriers at the compound were supporting his communications with other members of al Qaeda. I’m not going to comment on specifics. We’re obviously interested in any al Qaeda facilitator, to include couriers.”

Former Rep. Peter Hoekstra, Michigan Republican and onetime chairman of the House Permanent Select Committee on Intelligence, said tracking messengers became a prime mission early in the war on terrorism.

The CIA began collecting human sources instead of relying on phone intercepts prevalent in the 1990s when the intelligence budget was slashed.

“Tracking couriers has been an important part of the intelligence gathering for quite some time,” Mr. Hoekstra said.

“In 2001, the intelligence community was totally unprepared for the challenge that we saw from al Qaeda. In the nine or 10 years since then, they have significantly transformed themselves. … They are light-years from where they were on 9/11.”

He suggested that couriers have led the United States to other terrorist bosses.

“The deadliest job in the world for a while was becoming the No. 3 guy for al Qaeda,” said Mr. Hoekstra, referring to the killing of the group’s operations chief, Mohammed Atef, and then the demise of others who succeeded him.

“We got the No. 3 guy about every three or four months. … Part of our strategy was going after their command-and-control structure. Going after their hierarchy. That’s been a priority. The intel community uses its full range of tools to go after these guys.

“I can’t get into which one is the most efficient in terms of identifying and getting actionable intelligence. But, clearly, whether it’s electronic or whether it’s couriers the way they pass information, if you can penetrate the communication network, it’s a great tool in identifying who the decision-makers are and where they are so that you can act on it.”

An intelligence official said that in the hunt for bin Laden, the most difficult task was the gumshoe detective work.

“The hard work is finding the courier once he has been identified,” the official said.

“This would include the positioning of the best and the most unobtrusive surveillance team, meaning people who look and talk like people in Pakistan.

“It would mean finding a safe house without raising suspicions. It would include the best in clandestine photography as well. A really good surveillant is hard to find.”